The Importance of Chain Connectivity in the Formation of Non-covalent Interactions between Polymers and Single-walled Carbon Nanotubes and its Impact on Dispersion

Polymer nanocomposites have garnered incredible promise in the field of material science due to the excellent mechanical strength, thermal and electrical conductivities of the nanoparticles and the extension of these properties to the processing flexibility inherent to plastics. However, practical realization of these nanoparticle-based materials has been hindered by the tendency of these nanoparticles to aggregate as a result of strong inter-particle forces. In this dissertation, we investigate the formation of non-covalent charge transfer interactions between polymers and single-walled carbon nanotubes (SWNTs) with the goal of optimizing interfacial adhesion and homogeneity of nanocomposites without modifying the SWNT native surface.

Nanocomposites of SWNTs and three sets of polymer matrices with varying composition of electron donating or electron accepting functional groups were prepared. In the first part of this dissertation, quantitative characterization by optical microscopy and Raman spectroscopy and qualitative results through thick film composite visualization show that the existence of a moderate amount of interacting moieties along the polymer chain results in an enhanced intermolecular interaction with SWNT, which translates to an optimum nanoparticle homogeneity.

Calculations from density functional theory and Flory-Huggins theory correlate with the experimental results, which illustrate that chain connectivity is critical in controlling the accessibility of the functional groups to form intermolecular interactions. Thus, controlling the amount of interacting functional groups throughout the polymer chain such that an adequate distance between them is realized will direct the extent of charge transfer interaction, which enables tuning the SWNT dispersion.

The second part of this dissertation focuses on the elucidation of the morphology of these nanoparticle entities in a polymer matrix. The observance of microphase-separated peaks in the scattering patterns of polyacrylonitrile (PAN) nanocomposites indicate an ordering of the PAN polymer induced by the carbon nanotube cage, which could either be due to a thermodynamically bound layer around the SWNT or the occurrence of SWNT-induced PAN crystallization.

Finally, UV-Vis measurements were performed on SWNT-polymer suspension in order to comprehend the interactions that occur during nanocomposite fabrication. These results demonstrate that SWNT dispersions in pure N,N-dimethyl formamide (DMF) are stabilized by the adsorption of polymers onto the SWNTs.